The construction of a video camera using a conventional digital signal processing will be described with reference to FIG. 1. It should be noted that for the convenience of the explanation of the video camera, red (R), green (G) and blue (B) signal channels (will be referred to simply as R, G and B channels, respectively) sharing the same processing circuit, for example, will be identified with a common reference to which R, G and B are suffixed, respectively. The video camera is disclosed in the Japanese Unexamined Patent Application Nos. 46576 of 1997 and 65935 of 1980.
In FIG. 1, the conventional video camera is generally identified with a reference 501. As shown, the video camera 501 includes a pickup lens 511, color separation prism 512, three solid-state image sensing devices, 513R, 523G and 513B, such as a charge-coupled device (CCD) provided for R, G and B channels, respectively, three correlated double sampling circuits (CDS) 514R, 514G and 514B provided for the R, G and B channels, respectively, analog signal processing circuits 515R, 515G and 515B for the R, G and B channels, respectively, and analog-to-digital (A/D) conversion circuit (A/D converter) 516R, 516G and 516B provided for the R, G and B channels, respectively.
The video camera 501 also includes up converters 517R, 517G and 157B provided for the R, G and B channels, respectively, a linear matrix circuit 518, a kneeing circuit 519, an image enhancer 520, edge correction addition circuits 521R, 521G and 521B provided for the R, G and B channels, respectively, a gamma correction circuit 522, clipping circuits 523R, 523G and 523B provided for the R, G and B channels, respectively, and a signaling system changeover circuit 524.
Once the video camera 501 is put into action, light from an object is incident upon the pickup lens 511. The incident light is separated by the color separation prism 512 into three primary colors R, G and B. The color separation prism 512 has accurately fixed thereto the CDDs 513R, 513G and 513B corresponding to the primary colors R, G and B. The primary lights R, G and B separated by the color separation prism 512 are incident upon the CCDs 513R, 513G and 513B, respectively. Output signals from the CCDs 513R, 513G and 513B, indicating the primary colors R, G and B, respectively, are supplied to the CDS circuits 514R, 514G and 514B where the respective signals R, G and B will subjected to the correlated double sampling to generate analog video signals, respectively. The analog video signals are supplied to the analog signal processing circuits 515R, 515G and 515B, respectively, where they will be amplified and subjected to various analog signal processing including black-and-white (B/W) balancing, B/W shading, flare correction, etc. The analog video signal outputs from the analog signal processing circuits 515R, 515G and 515B are supplied to the A/D converters 516R, 516G and 516B, respectively, where the analog video signals will be sampled at a predetermined rate and converted into digital video signals, respectively. The digital video signal outputs from the A/D converters 516R, 516G and 516B are supp/lied to the up converters 517R, 517G and 517B and image enhancer 520.
Each of the up converters 517R, 517G and 517B includes a zero insertion circuit and interpolating filter to raise the sampling rate for digital video signals and re-sample the digital video signals. Normally, the sampling frequency used in the A/D conversion is the same as the horizontal drive frequency of the CCDs 513R, 513G and 513B. The digital signal processing in the video camera includes nonlinear processing such as the kneeing, gamma correction, B/W clipping, etc. which will be described later. Such nonlinear processing of a video signal will result in distortion of the signal waveform and occurrence of a harmonic whose frequency is several times higher than a frequency included in the video signal. When the harmonic component thus developed exceeds the Nyquist frequency (a half of the sampling frequency), it will return to a band of lower frequencies and become a return noise which will deteriorate the image quality. To avoid this, the Nyquist frequency is raised and the digital video signals are re-sampled with a higher frequency by the up converters 517R, 517G and 517B to prevent the return noise from occurring. The digital video signal outputs from the up converters 517R, 517G and 517B are supplied to the linear matrix circuit 518.
The linear matrix circuit 518 makes color correction of the video signals to correct color reproduction error caused by the fact that the imaging characteristic of the CCD is different from the ideal one. The video signal outputs from the linear matrix circuit 518 are supplied to the kneeing circuit 519.
The kneeing circuit 519 makes gradation compression of high-luminance portions of the video signals according to a predetermined kneeing characteristic. The video signal outputs from the kneeing circuit 519 are supplied to the addition circuits 521R, 521G and 521B.
The image enhancer 520 extracts high frequency components from the video signals and multiplies the high frequency components by a predetermined gain to thereby produce edge-enhancement signals for enhancing the edges of the video signals. The edge-enhancement signal outputs from the image enhancer 520 are supplied to the addition circuits 521R, 521G and 521B.
The addition circuits 521R, 521G and 521B are provided to add edge-enhancement signals to the video signals to shape the waveforms of the video signal edges. The video signals having the waveforms thereof thus shaped by the addition circuits 521R, 521G and 521B are supplied to the gamma correction circuit 522 where the video signals will be subjected to gamma correction. The video signal outputs from the gamma correction circuit 522 are supplied to the clipping circuits 523R, 523G and 523B. The clipping circuits 523R, 523G and 523B clip the signals whose levels are over the standard television video signal level. The video signal outputs from the clipping circuits 523R, 523G and 523B are supplied to the signaling system changeover circuit 524. The signaling system changeover circuit 524 converts the video signals into ones necessary for the system configuration of the video camera 501, such as NTSC- or PAL-based analog composite video signal, analog component video signals such as Y (luminance) signal, R-Y signal and B-Y signal, serial digital video data or the like.
Next, the image enhancer 520 will be described below with reference to FIG. 2.
As shown, the image enhancer 520 includes three vertical filters 531R, 531G and 531B provided for the R, G and B channels, respectively, horizontal-component up converters 532R, 532G and 532B provided for the R, G and B channels, respectively, vertical-component up converters 533R, 533G and 533B provided for the R, G and B channels, respectively, a horizontal-component R/G/B mixing circuit 534, a vertical-component R/G/B mixing circuit 535, horizontal edge enhancement signal generation circuit 536, a vertical edge enhancement signal generation circuit 537, an addition circuit 538, and a gain control circuit 539.
The digital video signals on the R, G and B channels branched from the A/D converters 516R, 516G and 516B are supplied to the vertical filters 531R, 531G and 531B, respectively, of the image enhancer 520. That is, the digital video signal on the R, G and B channels branched from the A/D converter 516R is supplied to the R-channel vertical filter 531R, the digital video signal on the G channel branched from the A/D converter 516G is supplied to the G-channel vertical filter 531G, and the digital video signal on the B channel branched from the A/D converter 516B is supplied to the B-channel vertical filter 531B.
As shown, the R-channel vertical filter 531R includes a first 1H delay circuit 541R, second 1H delay circuit 542R, vertical lowpass filter (LPF) 543R, and a vertical highpass filter (HPF) 544R. The first 1H delay circuit 541R delays the R component one horizontal sync period (1H), the second 1H delay circuit 542R is supplied with the R component having been delayed the one horizontal sync period (1H) by the first 1H delay circuit 541R, and will delay the R component a further horizontal sync period (1H). Namely, the second 1H delay circuit 542R outputs the R component delayed a total of two horizontal sync periods (2H). The vertical LPF 543R and vertical HPF 544R supplied with an R component not delayed (0H delay), R component delayed one horizontal sync period (1H) and an R component delayed two horizontal sync periods (2H). The vertical LPF 543R makes vertical lowpass filtering of the R components based on the R components having the 0H delay, 1H delay and 2H delay, respectively, to extract vertical low-frequency components from the R components. The vertical HPF 544R makes vertical highpass filtering of the R components based on the R components having the 0H delay, 1H delay and 2H delay, respectively, to extract vertical high-frequency components from the R components. That is, since the edge of a picture appears as high frequency components of the video signals, the vertical LPF 543R will cut off vertical edge component from the video signal, while the vertical HPF 544R will extract vertical edge components from the video signals. The R component output from the vertical LPF 543R is supplied to the R-channel horizontal-component up converter 532R, while the R component output from the vertical HPF 544R is supplied to the R-channel vertical-component up converter 533R.
To process data synchronously with the up converter 517R provided in a main video signal path, the horizontal-component up converter 532R make up-conversion of the sampling frequency for the R component similarly to the up converter 517R. Similarly, the vertical-component up converter 533R makes up-conversion of the sampling frequency of the R component.
Note that the G-channel vertical filter 531G and B-channel vertical filter 531B are constructed similarly to the R-channel vertical filter 531R and thus will not be described any more. Also, since the G- and B-channel horizontal-component up converters 532G and 532B and G- and B-channel vertical-component up converters 533G and 533B are constructed similarly to the R-channel up converter and thus will not be described any further.
The horizontal-component R/G/B mixing circuit 534 is supplied, from the R-, G- and B-channel horizontal-component up converters 532R, 532G and 532B, with R-, G- and B-component video signals having vertical high-frequency components cut off therefrom. The horizontal-component R/G/B mixing circuit 534 mixes the R, G and B components at a predetermined mixing ratio to generate a luminance component. That is, the horizontal-component R/G/B mixing circuit 534 outputs a luminance signal having vertical high-frequency components cut off therefrom. The luminance signal from which the vertical high-frequency components have been cut off is supplied to the horizontal edge enhancement signal generation circuit 536.
The vertical-component R/G/B mixing circuit 535 is supplied, from the R-, G- and B-channel vertical-component up converters 533R, 533G and 533B, with R-, G- and B-component video signals indicative of the extracted vertical high-frequency components. The vertical-component R/G/B mixing circuit 535 mixes the R, G and B components at a predetermined mixing ratio to generate a luminance component. That is, the vertical-component R/G/B mixing circuit 535 outputs a luminance signal indicative of the extracted vertical high-frequency components. The luminance signal indicative of the extracted vertical high-frequency components is supplied to the vertical edge enhancement signal generation circuit 537.
As shown, the horizontal edge enhancement signal generation circuit 536 includes a horizontal bandpass filter (BPF) 551, gain control circuit 552 and a level limiter 553.
The luminance signal output from the horizontal-component R/G/B mixing circuit 534 is supplied to the horizontal BPF 551 where horizontal high-frequency components will be extracted from the luminance signal. That is, the horizontal BPF 551 outputs a signal indicative of only the extracted horizontal high-frequency components. In other words, the horizontal BPF 551 will output edge components indicating only a horizontal edge. The horizontal edge component outputs from the horizontal BPF 551 are supplied to the gain control circuit 552. The gain control circuit 552 multiplies the extracted horizontal edge components by a predetermined gain control coefficient to control the extent of the horizontal edge enhancement. The output from the gain control circuit 552 is supplied to the level limiter 553 which will make signal level limitation of the horizontal edge components higher than a predetermined signal level and output a final horizontal edge enhancement signal.
As shown, the vertical-component edge-enhancement signal generation circuit 537 includes a horizontal lowpass filter (LPF) 554, gain control circuit 555 and a level limiter 556.
The luminance signal output from the vertical-component R/G/B mixing circuit 535 is supplied to the horizontal LPF 554 where horizontal low-frequency components will be extracted from the luminance signal. That is, the horizontal LPF 554 outputs a signal indicative of only the extracted vertical high-frequency components. In other words, the horizontal LPF 554 will output edge components indicating only a vertical edge. The vertical edge component outputs from the horizontal LPF 554 are supplied to the gain control circuit 555. The gain control circuit 555 multiplies the extracted vertical edge components by a predetermined gain control coefficient to control the extent of the vertical edge enhancement. The output from the gain control circuit 555 is supplied to the level limiter 556 which will make signal level limitation of the vertical edge components higher than a predetermined signal level and output a final vertical edge enhancement signal.
The horizontal and vertical edge enhancement signals thus generated are supplied to the addition circuit 538 and gain control circuit 539.
The addition circuit 538 adds the horizontal and vertical edge enhancement signals together to mix the horizontal and vertical directions, and the gain control circuit 539 makes a final gain control of the edge-enhancement signals whose horizontal and vertical directions have thus been mixed to output a final edge-enhancement signal.
The edge-enhancement signal generated by the image enhancer 520 as above is added to a main video signal. The video signal having such an edge-enhancement signal added thereto becomes an edge-enhanced picture well defining the edge of an object.
It is well known that when a pulse-shaped rectangular wave signal is supplied to an analog amplification circuit composed of a transistor, operational amplifier or the like, the through rate of the amplification circuit at the rise is different from that at the fall of the signal. It is also known that a signal having passed through an analog filter composed of a resistor, capacitor, coil and the like will have the phase characteristic thereof distorted. Further, it is known that on the wires in a printed wiring board, a signal will be distorted by reflection or the like. For the above reasons, the waveform characteristic is distorted at the input and output of an analog circuit. For example, a rectangular wave signal as shown in FIG. 3A, after having passed through an analog amplifier or filter, will not show the same waveform characteristic at the rise and fall thereof because of a difference in through rate of the analog amplifier, for example, between at the rise and at the fall of the signal as shown in FIG. 3B.
Generally, in a video camera using the digital signal processing, light signal is acquired as an analog electrical signal from a CCD and subjected to A/D conversion to output a digital video signal. Therefore, an analog circuit has to be provided between the CCD and A/D conversion circuit. The analog circuit is composed of an active device such as a transistor, operational amplifier or the like and a passive device such as a resistor, capacitor, coil or the like, mounted on a printed wiring board. Therefore, a video signal picked up by the CCD will be distorted specifically to the analog system under the influence of the analog circuit and be converted, as stilled distorted, into digital data.
If the original signal having the waveform thereof distorted through the analog circuit as shown in FIG. 3B is passed to the image enhancer, there will be generated an edge-enhancement signal having the distortion of the extracted original signal waveform as shown in FIG. 3C. Addition of the distorted edge-enhancement signal to the original signal waveform will result in a signal whose distortion is rather enhanced as shown in FIG. 3D and thus in a picture remarkably distorted. Such a waveform distortion is a big problem to solve for a video camera, especially a broadcasting service-directed one for which a high image quality is required.
Thus, the edge enhancement should desirably be done of video signals after correction of their waveform having been distorted through the analog circuit.
To eliminate such a distortion, a waveform distortion due to the analog signal processing could be approximated by a digital filter and corrected by applying an inversion of the approximated distortion.
However, the waveform distortion caused by a difference in through rate of an analog amplifier between the rise and fall of the waveform is on the order of a few nanoseconds (ns) and equivalent to a length of time several to tens times shorter than a sampling period used for the digital signal processing in the video camera. Therefore, even if it is tried to approximate, by a digital filter, a distortion caused in an analog signal processing and apply an inversion of the approximated distortion, for example, in order to limit the distortion, the sampling frequency has to be raised several to tens times to configure such a digital filter, which will lead to a complicated circuit configuration and increased power consumption. Namely, it is very difficult to implement this method of distortion elimination.